1. Field of the Invention
The present invention relates to a method of analyzing a composition depth profile of a solid surface layer using X rays. The method is especially suitable for an evaluation of composition depth profiles of gate insulating films of transistors (Tr), which are used as portions of semiconductor integrated circuits.
2. Description of Related Art
As semiconductor devices (Large Scale Integrated Circuit, LSI) become finer, ultrathin nitride-oxide films (1 nm to a few nm) are beginning to be introduced as gate insulating films of transistor cells. Functions of gate insulating films depend to a large degree on densities and distributions of nitrogen (N) in the nitride-oxide films. Therefore, film qualities of such nitride-oxide films have been studied using the secondary ion mass spectrometry (SIMS) and the X ray photoelectron spectroscopy (XPS) in recent years (for example, see R. I. Hedge, B. Maiti, and P. J. Tobin: J. Electrochem. Soc., Vol. 144, No. 3, March 1977, pp. 1081-1086).
SIMS is a technique characterized by including steps of causing primary ions to impinge on a sample to sputter it, and analyzing in mass and detecting the sputtered secondary ions. In the case where a nitride-oxide film as described above, wherein N is distributed in a very shallow region in the film, is analyzed using SIMS, the following effects will arise: the mixing effect of the primary ions (an effect such that the primary ions incident on a sample mix the target elements to be detected and the matrix elements of the sample in a region up to a certain depth from the surface thereof); and the knock-on effect (an effect such that an impact energy of primary ions cause the target elements to be forced to the direction of the depth).
In the case of using SIMS to analyze nitride-oxide films, the mixing effect and the knock-on effect make a detected density of nitrogen lower than the actual density and diffuse nitrogen atoms in the sample around the measured region to spread the distribution of the nitrogen atoms in analyzing it. As a result, the density and distribution of N with respect to the direction of the film thickness can not be measured accurately.
In contrast, XPS is an analyzing technique characterized by steps of irradiating a sample with X rays, and spectroscopically analyzing the excited photoelectrons to determine the intensities of detected elements. Especially, the angle-resolved XPS is used to measure the distributions of elements in a thin film (for example, see (1) M. Pijolat and G. Hollinger, Surface Science 105, 1981, pp. 114-128, and (2) Ryu, et al., Dai 49-kai Ouyo-busuri Kankei-rengo Koenkai (49th lecture meeting of applied physics-related alliance), the collection of preliminary reports of the lectures p.815). The evaluation of the density distributions of elements in the direction of the film thickness using the angle-resolved XPS utilizes a principle such that the intensities of photoelectrons created at a depth (x) below the sample surface are exponentially attenuated according to the relation shown by the following equation (1) when the photoelectrons reach the sample surface.                               I          ⁡                      (            x            )                          =                                            I              0                        ⁡                          (              x              )                                ⁢                      exp            ⁡                          (                                                -                  x                                                  λ                  ⁢                                      xe2x80x83                                    ⁢                  sin                  ⁢                                      xe2x80x83                                    ⁢                  θ                                            )                                                          (        1        )            
where x is a depth below the sample surface (solid surface), xcex represents an inelastic scattering mean free path of photoelectron, xcex8 represents an angle which the sample surface forms with the detector and X rays, I0 represents an intensity of photoelectrons created at a depth x below the sample surface, and Ix represents an intensity of photoelectrons created at a depth x when the photoelectrons reach the sample surface. As shown in Equation (1), when making the angle xcex8 smaller, the path which photoelectrons created in the sample travel along to reach the sample surface becomes longer to increase the attenuation in photoelectron intensity. On the other hand, when making the angle xcex8 larger, the path becomes shorter and thus the attenuation in photoelectron intensity becomes smaller.
More specifically, the density distributions of elements in the direction of the film thickness have been determined by the angle-resolved XPS including the steps of: assuming density distributions of target elements to be detected in the film; calculating the detected intensities of the elements when the sample is irradiated with X rays at a variety of angles xcex8; and repeating the step of assuming the density distributions of the elements until the photoelectron intensities calculated by the least squares method converge to the actually-measured photoelectron intensities.
However, the conventional method using the angle-resolved XPS has no selectivity to cause calculated intensities of photoelectrons to converge to the actually-measured intensities effectively and efficiently and thus the method has required much time to conduct the calculation to determine a composition depth profile of a solid surface layer.
The invention was made to solve the above-described problem. In other words, it is an object of the invention to provide a method of analyzing a composition depth profile of a solid surface layer, which enables us to determine the density distributions of elements in a solid surface layer in the direction of the film thickness simply and accurately for a short time. The method is realized by the addition of selectivity to cause photoelectron calculated intensities to converge to the actually-measured intensities effectively and efficiently.
The invention can solve the above problems. In other words, the invention provides a method of analyzing a composition depth profile of a solid surface layer, wherein actually-measured intensities of photoelectrons emitted from the solid surface layer by irradiating the solid surface layer containing at least two or more species of elements with X rays and photoelectron calculated intensities obtained by making a calculation assuming an elemental composition ratio for each of a plurality of sub-layers into which the solid surface layer has been temporarily divided are utilized to determine the composition depth profile of the solid surface layer. The method of the invention includes the step of at least repeating an approximate calculation including: distinguishing a specified sub-layer such that the calculated intensities best converge to the actually-measured intensities in the sub-layers; and correcting elemental composition ratios at least for the specified sub-layer so that the calculated intensities converge to the actually-measured intensities, thereby determining the composition depth profile of the solid surface layer.